1. Field of the Invention
The present invention relates to color management systems in which metamerism is reduced when a destination image (typically, a color printout) is viewed under different viewing conditions (such as different illuminants or different surrounds).
2. Description of the Related Art
“Metamerism” describes the physiological response of the human visual system by which exactly the same color appearance can result from different color stimuli. It is because of metamerism that modern four-color separation processing can reproduce such a broad range of colors.
There are some objects that appear the same to the human eye, and yet the energy spectra reflecting back from these objects are very different. Without metamerism, printing a picture containing each of these objects would require special inks for each object. The number of inks required would be unmanageable. Because of metamerism, almost the same stimuli for the eye can be produced using only a few different inks, usually cyan, magenta, yellow and black.
Again because of metamerism, it is often possible to produce the same color appearance with different combinations of CMYK colors. For example, a color formed with 100% cyan, 100% yellow, 30% magenta and no black would look the same as a color formed with 70% cyan, 70% yellow, 30% black, and no magenta. This feature of metamerism is exploited in the invention described later.
Metamerism is fundamentally affected by the viewing conditions such as the illuminant and the surround, but particularly the illuminant. As an extreme example, consider a four-color printout of a green leaf side-by-side with an actual leaf. In natural light, both appear green. However, when viewed under a yellow light, while the leaf will still appear green, the four-color print will appear black or dark blue.
Conventional color management systems ignore the effects of a changed illuminant (or other changes in the viewing conditions) on the destination image. The flow of a typical color management system is illustrated in FIG. 14. Apart from gamut mapping and other changes, FIG. 14 is somewhat similar to the process described in U.S. Pat. No. 5,463,480. The process operates on each individual color in a source image 150 so as to produce a corresponding CMYK color value for a destination image 160 in which the appearance of the colors in the destination image accurately matches the appearance of the colors in the source image. Thus, in step S1401, each individual RGB color value in the source image is converted into an XYZ tristimulus value, and a forward appearance model is applied in step S1402 for viewing conditions at the source so as to convert the XYZ tristimulus values into a JCh value in JCh color space (or other perceptual color space). In step S1404, gamut adjustments are made in the perceptual color space, and the resulting JCh color value is inverse-transformed in step S1405, using the destination device viewing conditions, into a single XYZ tristimulus value in viewing condition dependent color space. The XYZ tristimulus value is then converted to a color value in output device coordinates (such as CMYK), for printout by the output device (step S1407).
The resulting CMYK color, when viewed under the viewing conditions used in the inverse transform, will appear as an accurate match to the original color in the source image. As noted above, the resulting color will exhibit metameric shifts if viewed under viewing conditions that differ from that applied at step S1405 in the inverse-transform.